A standard binarization/Variable Length Coding (VLC) codeword assignment scheme used in Advanced Video Coding (AVC) is shown in Table 1. In AVC, the prediction mode for an intra coded block is signaled in the bitstream to the decoder. Possible chroma prediction modes/directions are vertical, horizontal, DC, diagonal and “same as luma.” The modes are each assigned a codeword from 0 through 4 and are signaled using truncated unary code in the bitstream.
TABLE 1Intra prediction mode for chroma component.intra_pred_mode_chromaBinarization/VLC Codeword0 (vertical)01 (horizontal)102 (DC)1103 (diagonal down right)11104 (same as luma)1111
A statistical study shows that there is a high probability for the chroma components to use the sample prediction direction as the luma component of the block. However, the “same as luma” mode is assigned the longest codeword in Table 1 (4 bits), which is not efficient. On the other hand, redundancy also exists between mode 4 and mode 0-3, when the luma prediction mode is one of vertical, horizontal, DC and diagonal down right. Therefore, a better design of the codeword table is needed.